Frequency control system



Jan. 8, 1957 H. A. ROBINSON 2,777,064

FREQUENCY CONTROL SYSTEM Filed Dec. 11, 1953 s sheets-sheet 1 m F/,rfp /M Uff/7 Q2/WJ F571. 'Z mmf/:f 45E/ra j if; fff- Fm 7055 f/''fYM-J 4 fao/1%) @55g l ('/0 g i/ k :gyn Marc. /j J MAZ'. l a, y w w /0 (wav/vm) (zz mi) v 4 i Z7 fan. f/rfa. Li L Til/15 111111 l 1111 11 111A I7 INI/ENTOR.

Jan. 8, 1957 H. A. RoBlNoN FREQUENCY CONTROL SYSTEM Filed Dec. ll, 1953 3 Sheets-Sheet 2 TUE/F pwfLfffe/Zeff@ .60A/c. 54h/mow Fia/ff MaJ/#mg ifa INI/ENTOR.

N w M Jan. 8, 1957 H. A. RoBlNsQN FREQUENCY CONTROL SYSTEM 3 Sheets-Shea?l 5 Filed Dec. ll, 1953 INI/ENTOR FREQUENCY CONTROL SYSTEM Harris A. Robinson, Palmyra, N. J., assignor, by mesne assignments, to the United States of America as rep resented by the Secretary of the Air Force Application December 11, 1953, Serial No. 397,572

12 Claims. (Cl. Z50-36) This invention relates .to a frequency control system, and more particularly to a contro'l system for a mul-tichannel controlled or captive oscillator.

In my copending application, 'Serial No. 257,\l48, tiled November 19, 195'1, there is .disclosed la multichannel ('44,000-.channel) frequency generator and frequency contro'lsyste-m for a master (captive) oscillator which is used as a heterodyne oscillator in a communications transmitter-receiver. The frequency generator disclosed in such application operates to permit the selection .of any one of 44,000 possible communication channels and to thereafter automatically :tune the master oscillator to a frequency such th-at communication may ibe carried on in the desired, .selecte-d channel.

ln the referenced frequency control system, between successive automatic tuning cycles or during normal operation of the transmitterareceiver for communications purposes the frequency ofthe master oscillator is stabilized or controlled by means of Ia phase discriminatorreactance tube combination (the reactance tube, incidentally, l'being used also during a port-ion of .the automatic tuning cycle), a voltage `wave representative of the lmaster oscillator output frequency being compared in the phase discriminator with a voltage wave of stable reference lfrequency and the output Vof the p` fase discriminator being applied to thev reactance tube associated with the master oscillator. With this .type of frequency control or stabilization means, there is normally a rather extended hold-in ran-ge over which the master oscillator, once :locked in to the desired frequency selected fby the channel `selecting means, will hold in under frequency control by the phase discriminator-reactance tube combination. However, this hold-in range is effective only for relatively lslow frequency variations or drifts of the master oscillator, those due to the effects of temperature, humidity or voltage variations, .aging of tubes, etc. rThere is a much more limited lock-in range contained within the holdin range, this llocleiny range rbeing characterized by the ability of the frequency lgenerator and control system to initially (i. e., during 'an automatic .tuning cycle) lock in the frequency of the captive master oscillator, or -to regain frequency control of the mast-er :oscillator lafter :a transient disturba-nce (e. g., arising from mechanical shock or from a vol-tage surge). Such transient disturbances lare very likely to occur in aircraft, lfor which the referencedwtransmitterreceiver, including the frequency generator and control system, is particularly adapted. When Ia frequency generator system of the type disclosed in my aforesaid copending applic-ation is to be subjected to such transient disturbances, the reliability of the system is directly proportional to :the extent `of the lock-in range. FIlhe present invention is an improvement over the system of my cop ending application.

`An object of this invention" is to devise a phase discriminator-reactlance tube type lof frequency control system wherein the lock-in range is enlarged many fold as compared to systems of the prior art.

vcopending'- application.

2,7 7 7,064 Patented Jan. s, 1957 ICC Another object is'to provide a multichannel frequency generator for a master controlled .or captive oscillator, in which the master oscillator is automatically relocked to the proper frequency or phase after it has 'been Iforced out of lock as a result of a severe disturbance, electrical or mechanical.

A further object is to devise a frequency generator and .control systern'fora cont-rolled oscillator, in which the .controlled oscillator isfautomatically rclocked Ito .the

proper frequency after it has fallen out oflock as the result of atransient dist-urbance,'in such situations Where the controlled oscillator is outside the normal lock-in range of the frequency control ysystem after the disturbance has ceased to exist.

r[lhe objects of this invention are accomplished, briefly, in one exemplilication of the invention in the following manner: the frequency generator or frequency cont-rol 'system for the'master'oscillator of ya transmitter-receiver includes a plurality of i-cascadedmixers in the first: of which the master (captive) oscillator frequency :is mixed with la selected .crystal-stabilized frequency land in the subsequent ones of which the'variou-s result-ing beat frequencies `are 'mixed with respective selected crystal-:sta-

lbilized frequencies. Following 1the final mixer, in wlhich a lixed beatfrequency e. g., 500 kc.) is produced, a regenerative-type frequency 'divider is `used to divide this frequency 'down to 50 kc. to provide one input to a phase discriminator. Any interruption or I:loss of normal frequency control (i. e., ybreaking out of lock) .of the captive oscillator' results in "a marked cha-nge in the grid voltage of a tube in this divider, and in one :embodiment this change-is utilized, as abias `voltage, to gate on -a-n `auxiliary low 'frequency oscillator which modulates or sweeps the frequency 'of the master oscillator -by means of' the reactance tube used for normal frequency cont-rol. During such sweep, the master `oscilator Vwill lookin again. 'In the preferred specific embodiment, no `'auxiliary low frequency oscillator is utilized but the regenerative frequencydivider itself Ibrealcs into 'an oscillation in response to the loss'of normal frequency control of the 'masterv oscillator and this oscillation acts through lthe phase discriminator (used 'for normal frequency control) and reacta-nce tube to cause :a fluctuation ofthe master oscillator frequency during "which the master oscillator will lock in again and normal frequen-cy controlwill be restored.

The foregoing'and' other objects of the invention |will be better understood from the following description of some exemplitications thereof, reference lbe-ing had tothe 'accompanying drawings, wherein:

- generator or control system modified in accordance with the teaching of one embodiment of this invention, vthe master captive oscillator 1` isthe oscillator that is automatically controlled in frequency by vthe frequencygenerator or control system illustrated, and the output of this oscillator is utilized for heterodyning purposes in the transmitter-receiver- (not shown) with which the system illustrated is associated. The transmitter-receiver may for example be arranged as disclosed in my aforesaid The master (captive) oscillator 1 is arranged to be permeability tuned and has an output frequency of 1.9 to 12.9 mc. (in several bands), as indicated. Exact frequency control of oscillator 1 is obtained by means of reactance tube 2 coupled to oscillator 1.

The frequency control system illustrated utilizes har monic generators excited fromta crystal-stabilized oscillatory source. The heart of the unit which acts as the oscillatory source is a 500-l c. reference crystal-controlled oscillator 3 which is extremely stable. Output of 500 kc. from reference oscillator 3 drives a 500-kc. harmonic generator 4. Generator 4 is preferably of the two-stage type described and claimed in my copending application, Serial No. 253,141, led October 25, 1951. A Thousands selection switch 5 has twenty-two positions and is mechanically coupled to a frequency selecting means in generator 4 so that any selected one of the sixth through twenty-seventh harmonics of the 500-kc. input to generator 4 may be passed from said generator to No. 1 mixer 6, depending upon the position of switch 5. Output from the master oscillator 1 is also supplied to the mixer 6 and this oscillator frequency, beating with the output frequency of generator 4 insuch mixer, produces a difference frequency mixer output which may vary from 600 to 1100 kc., depending upon the frequency selection switch settings.

The 500-kc. output of oscillator 3 drives a series of cascaded locked-in oscillator `frequency dividers, beginning with a G-kc. locked-in oscillator 7 the output of which drives a 50-kc. stage 8 whose output, in turn, drives a 5-kc. stage 9. The 50-kc. stage 8 includes amplifier and pulse Shaper circuits whereby 50kc. pulses and a 50l-:c. sawtooth wave may be derived from this stage for utilization in circuits to be later described.

The 600-1100 kc. difference frequency output of mixer 6 ispassed through a bandpass filter 10 to provide one of the inputs to No. 2 mixer 11, the other input being provided from a 50kc. harmonic generator 12. The generator 12 is supplied with 50-kc. pulse input derived from divider stage 8 over lead 32 and harmonics of this input frequency lying in the range of 450 to 900 kc. are selected by the Hundreds selection switch 13, which has ten positions. The particular harmonic of 50 kc. selected at the output of generator 12 depends of course upon the position of switch 13, and this selected harmonic is passed on to mixer 11 as input to mix with signal from filter 10. The selective .circuit in filter 10 is tuned approximately by the Hundreds switch 13.

Output from mixer 11 is transferred, through the selectivecircuit bandpass filter 14, tunable inr ten steps between 150 and 200 kc. as the Tens switch 15(which has ten positions) determines, to No. 3 mixer 16. A 5-kc. harmonic generator 17 is supplied with 5-kc. input derived from divider stage 9 and harmonics of this input frequency lying in the range of 35 to 80 kc. are selected by the Tens switch 15. The particular harmonic of 5 kc. which is selected by switch 15 from generator 17, is passed on to mixer 16 as input to mix with signal from filter` 14.

Output from mixer 16 is transferred through the bandpass filter 18, which passes a frequency band from 230 to 235 kc., to No. 4 mixer 19. The Units switch 20, which has twenty positions, selects clystals in crystal oscillator units 21 and 22. One of the group of four crystals from 120.0 `to 120.75 kc. in oscillator 22 is selected, while one of the group of tive crystals from 145 to 149 kc. in oscillator 21 is selected; The crystals in oscillator 22 have frequencies of 120.0, 120.25, 120.5 and 120.75 kc., While those in oscillator 21 have frequencies of 145, 146, 147, 148 and 149 kc. The outputs of thettwo crystal oscillators 21 and 22 excite No. 5 mixer 23, the switching actuated by Units switch being arranged to produce output from mixer 23 of any one of twenty frequencies, spaced every 250 cycles in the range from 265 to 269.75 kc. A bandpass filter 24 couples this mixed crystal output to No. 4 mixer 19.

The output of No. 4 mixer 19 is nominally 500 kc. In

other words, as the master oscillator 1 is scanned through a band of frequencies there will be one segment of the oscillator tuning range, corresponding to the settings of the switches 5, 13, 15 and 20 (which determine the selected frequencies fed to the several mixers) where a signal near 500 kc. will be developed in the output of mixer 19; this signal output in the vicinity of 500 kc. corresponds closely to the desired correct tuning of the master oscillator 1. A specific numerical example will make this clearer. Suppose that the master oscillator frequency is 3,462.5 lic. Then, the ninth harmonic of 500 kc. is selected in harmonic generator 4 and this 4500-ltc. frequency combines in mixer 6 with the 3,462.5-kc. output of the master oscillator 1, giving a difference frequency of 1037.5 kc. which is passed through filter 1t) to mixer 11. The seventeenth harmonic of 50 kc., which is 850 kc., is selected from harmonic generator 12 and this frequency combines with the 1037.5 kc. frequency in mixer 11 to give a dilference frequency of 187.5 kc. which is passed through lter 14 to mixer 16. The ninth harmonic of 5 kc., which is 45 kc., is selected in harmonic generator 17 and this frequency combines with the 187.5- kc. frequency in mixer 16 to give a sum frequency of 232.5 kc. which is passed through filter 18 to mixer 19. In the oscillator 22, a frequency of 120.5 lrc.,is selected, while in the oscillator 21 a frequency of 147 kc. is selected. These latter two frequencies are mixed in mixer 23 to give a sum frequency of 267.5 kc. which is passed through lter 24 to mixer 19. This 267.5-kc. frequency combines with the 232.5-kc. frequency in mixer 19 to give a sum frequency of 500.0 kc. which is passed through a selective filter Z5 (tuned to 500 kc.) to a mixer 26, in the latter to be divided down, in effect, to 50 kc., for example, which passes through a filter 27 to a phase discriminator 28. The mixer 26 constitutes part of a regenerative frequency divider 29 (indicated by the dottedline enclosure which is so labeled) which is located between the output of filter 25 and phase discriminator 28, and which functions to divide the 500 kc. input thereto (from filter 25) by ten, to provide a 50 kc. output for phase discriminator 28.

The harmonic generator-mixer arrangement herein described is the same as disclosed in my said copending application Serial No. 257,148. The arrangement described constitutes a multi-channel frequency generator, providing 44,000 frequency channels for the master oscillator 1, one channel every 250 cycles in the range extending from 1.9 mc. to 12.9 mc. Each frequency channel is selected by the setting of the four switches 5, 13, 15 and 20.

The 50-kc. output of divider 29 is coupled as one input to phase discriminator 2S, preferably through an amplifier and phase inverter arrangement (not shown). A 50-kc. sawtooth-shaped output derived from divider stage 8 over lead 33 is supplied as the other input to phase discriminator 28. In the phase detector or discriminator 28 a direct current control output results from the phase comparison of the 50-lrc. signal from filter 27 and the 50- kc. sawtooth signal derived from the reference 50-kc. source 8. The control output of the phase discriminator is direct coupled (preferably through a cathode follower stage, not shown) to the grid of the reactance tube 2 for the master oscillator 1, in order to correct for frequency drifts of the master oscillator 1.

The foregoing constitutes an automatic frequency control system for the captive master oscillator 1, by means of which the master oscillator is stabilized in frequency by a phase discriminator 28 which compares the heterodyned output of oscillator 1 (heterodyned through cascaded mixers 6, 11, 16, 19 and 26) with the divided output of the reference crystal oscillator 3 (divided through dividers 7 and S).

It is desired to be pointed out that the heterodyned frequencies originating from oscillator 1 and produced by the successive mixing steps in the mixers 6, 1,1, 16,

19 and 26 have to pass through all of vthe selective circuits 10, 14, 18 and 25. If there is 'a severe disturbance of a transient nature (due to a mechanical shock or a voltage surge, etc.) anywhere in the entire frequency control system, including all ofthe units 1-25, the 500- kc. signal may disappear entirely from the output of lter 25, or it may decrease to an unusable value, since due to the cascaded mixing arrangement described it is necessary that all signal-producing and `sigual-transferring units be operating properly, in order to produce a '500- kc. signal at the output of filter 25.

Even though the severe disturbancesmentioned in the preceding paragraph may be only transient, the master oscillator may not lock-in to the crystal-derived frequencies when the cause of the disturbance is removed or has ceased to exist. This will now be explained with reference to Figure 2. With a phase discriminator-reactance tube frequencycontrol of the type described, there is normally a rather extended hold-in range over which the master oscillator l, once locked in, ywill hold under frequency control. in other words, the phase discriminator 23 will operate to provide ay rather Wide range of reactance control volts (on each side of nominal frequency represented by zero voltage) which, applied to reactance tube 2, will operate to hold under control the frequency of the master oscillator 1. This extended holdin range is represented by the upper dotted line A in Figure 2, which extends over a rather wide range of React` y, Control Volts, on either side (plus and minus) of the zero volts or Nom Value.

This hold-in range, however, is effective only for relatively slow variations such as those resulting from temperature or humidity changes of oscillator components, slow voltage changes, tube aging, etc, Thus, for slow variations or frequency drift trends of the master oscillator ll, the frequency of such captive oscillator will be held in control, by means of the phase discriminatorreact'ance tube combination, over a rather wide range of reactance control voltages, as indicated by the dotted line A in Figure 2. There is a much more limited lockin range (indicated by the shaded rectangle B in Figure 2) contained within' the hold-in range A, this lockin range being characterized by the abilityV of the frequency Lgenerator or frequency control system of Figure l to initially (i. e., when the equipment is first turned on or when ay new 'channel is selected and therefore an automatic tuning cycle of the oscillator takes place) lock in the frequency of the master captive oscillator 1, or to relock the frequency of the master oscillator after a transient disturbance (due to a mechanical shock or a voltage surge, etc.). The phase discriminator 28 thus provides only a small range of reactance control volts B (and correspondingly limited control) which, applied to reactance tube 2, will operate to initially lock in the master oscillator 1 or to relock this oscillator after a transient disturbance.

Now, let us suppose that due to slow component drifts of the master oscillator from the initial lockin point, at which the React Control Volts is Zero or Nom., the reactance control voltage produced by phase discriminator 28, and effective on reactance tube 2, has a value suchy as to be outside the lock-in range B but well Within the hold-in range A, such a value as represented by pointC in Figurel 2, for example. This'is not at all an unusual situation. lf, now, there is a severe transient disturbance due to a mechanical shock or a voltage surge, frequency control of the master oscillator will be momentarily lost and the master oscillator will break out of lock, since the oscillator 1 will jump so far off its proper frequency (as a result of the disturbance) that it is outside the hold-in rangef A of the phase discriminator 28. When the cause of 4the disturbance is removed or has disappeared, even if the master oscillator then returns to approximately the same frequency it had immediately prior to the disturbance, control will not be re'locked, since the point C is outside the lock-in range B and since the reactance control voltage must lie within the range B in order to relock after a transient disturb-` ance. Thus, frequency control of the master oscillator 1 is lost and the frequency of such voscillator will shift around erratically, since due to the necessity of adding a relatively unstable reactance tube 2 thereto in order to obtain a precision frequency control, the kstability of the oscillator 1 is inherently impaired.

The foregoing description yand explanation applies to a frequency generator and control system not utilizing the present invention. The reliability of a frequency generator and control system subjected to such transient disturbances as may be encountered, particularly in aircraft, for which the system is particularly adapted, is thus directly proportional to the extent of the lock-in range, which is rather limited at B in Figure 2. The present invention enlarges thisflock-in range B many fold, to an extent such as illustrated at B', by the use of an oscillatory, sweep, or fluctuating voltage applied to` the reactance tube 2 at any time when frequency control of the master oscillator 1 is not lockedin. Thus, the upper dotted line A and the upper rectangle B (referred -to in the above description) are labeled with no sweep (since for these characteristics the instant invention is not utilized) and the lower dotted line Av (which is of the same extent as line A) `and the lower rectangle B' are labeled with sweep, since these characteristics represent the present invention, utilizing the aforementioned sweep voltage. f

According to the invention, the relatively low frequency sweep or fluctuating voltage which, as stated, is applied to the reactance tube 2 whenever the master oscillator 1 frequency is not lockeddnj sweeps the master oscillator frequency either side of nominal or zero reactance control volts, and during said sweep the master captive oscil later frequency passes through the cascaded series of mixers and associated bandpass filters 6, 10, 11, 14, 16, 118, i9, 2.5, 26 and 27, and reestablishes the frequency con trol or re-locks the master oscillator, the locking in of the master oscillator cutting onc the sweep voltage. The details of the operation of this sweep voltage will become apparent as the description proceeds.

The frequency divider 29, previously referred to briefly, is of the regenerative type. The SOO-kc. output from filter 25 is coupled to the regenerative frequency divider 29, comprising No. 6 mixer 26 the output of which is fed to a 50-kc. filter or output tank 27, the divider also including a frequency tripler 30 which receives output from filter 27, and a l50kc. filter or tank 31 which receives output from tripler 36 and transfers its -output signal to the input side of mixer 26, which also functions as a frequency multiplier to multiply the 15G-kc. signal received from filter 3l to a frequency of -450 kc. to beat with the 50G-kc. signal received from filter 25, thereby producing the 5() kc. required'for filter 27. This regenerative frequency divider operates, in effect, to divide the SGO-kc. signal at the output of filter 25 down to a frequency of 50 kc. at the output of filter 27. This provides a frequency division ratio of ten.

The preceding paragraph describes a preferred form of the regenerative frequency divider 29, which form will be referred to hereinafter in connection with the detailed circuit diagram of Fig. '5. However, in a regenerative divider now to be referred to in connection with Fig. 3,

According to a rst embodiment of this invention, an auxiliary lowfrequency oscillator 34 is controllably gated on and olf by means of a bias connection 35 `coupled to the mixer 26 (which, as previously stated, in this embodiment is only a mixer and not also a frequency multiplier) of regenerative frequency divider 29 and controlling a controllable gate 34 which is coupled between the output of oscillator 34 and the phase discriminator 28. Oscillator 34 preferably is an audio oscillator operating at a frequency on the order of 150 cycles per second. The output of oscillator 34 is applied through discriminator 28 to reactance tube 2, when gate 34' is open, to modulate or sweep the frequency of master oscillator 1 by the oscillatory wave output of auxiliary oscillator 34.

Auxiliary oscillator 34 may be of any type, producing either a sine Wave or a non-sinusoidal output (such as a sawtooth). The output waveform of this oscillator is not critical. However, the auxiliary oscillator frequency must be relatively low with reference to the time constant around the frequency control loop of the master oscillator.

Now referring to Fig. 3, which is a detailed schematic of the iirst embodiment of the invention including regenerative frequency divider 29, auxiliary oscillator 34 and gate 34', 50G-kc. output from bandpass tilter 25 is applied through a coupling capacitor, to the suppressor grid 39 of No. 6 mixer tube 26, which constitutes the tirst stage of regenerative frequency divider 29 and which is a pentode vacuum tube. A resonant filter circuit 27, tuned to 50 kc., is connected directly to the anode 41 of tube 26. The 50G-kc. signal applied to grid 39 of tube 26 is in effect divided down to 50 kc. at the anode 41 of tube 26, and this SO-kc. signal (selectively passed by 4tuned circuit 27) is taken oi from anode 41 by means of a coupling capacitor 42 and applied as one input to the phase discriminator 28,; preferably by way of a phase inverter (not shown in Fig. 3). This SO-kc. signal passing through capacitor 42 constitutes the output of frequency divider 29.

The SO-kc. signal appearing atanode 41 is also applied through a coupling capacitor 4t) to the control grid 43 of the frequency multiplier tube 39, which also is a pentode vacuum tube. Tube 30 is biased so that it operates as a frequency multiplier, multiplying by a factor of nine to produce a 150-kc. signal from the 50-kc. signal input thereto, this 45o-kc. `signal being selectively passed by means of a resonant filter circuit 31 connected directly to the anode of tube 30 and tuned to 450 kc. The 450- kc. signal appearing at the anode of tube 36 is applied, by way of a capacitive divider including two capacitors 47 and 48 connected in series across circuit 31', to the control grid 45 of tube 26.

As previously stated, in this embodiment of the invention tube 26 functions as a mixer, to mix the 450-kc. signal received from the anode of multiplier 30 (by way of filter 31') with the 50G-kc. signal received from lter 25, thereby producing a SO-lrc. signal at anode 41 which is transmitted on to phase discriminator 2S for utilization therein. The regenerative frequency divider described, during its normal operation, divides the 50G-kc. input sigualapplied to grid 39 down to a 50-kc. signal at the anode 41 (across tuned circuit 27), effecting a division by ten.

The SO-kc. output signal from divider 29 is applied to a phase detector or discriminator 28 (not shown in Fig. 3), for` phase comparison therein with a standard or reference SO-kc. signal. This latter signal has a sawtoothshaped waveform and is derived from the frequency divider stage 8 of Fig. l and is applied by lead 33 to the phase discriminator 28. The D. C. control output of the discriminator results from the phase comparison of the 50-kc. sawtooth reference wave and the Sti-kc. signal from divider 29. This D. C. control output is applied to the grid of the reactance tube 2 to control the frequency of the master oscillator 1. In this way, during normal operation of the transmitter-receiver` of this invcntion,`the phase discriminator 28 very accurately controls the frequency of master 1, a correcting voltage out of the phase discriminator appearing whenever oscillator 1 tends to drift in frequency; the frequency of this oscillator is then maintained very accurately at its proper value by the phase comparison with a highly stable crystal-controlled frequency. The phase discriminator 28 has the hold-in range A and the lock-in range B already described in connection with Fig. 2.

The characteristics of the regenerative frequency divider 29, comprising mixer 26 and multiplier stage or harmonic generator 30, are such that under normal frequency control of oscillator 1 (when conditions are normal, that is, when oscillator' 1 is being held in control so that there is a SOO-kc. signal of the proper magnitude at the output-of bandpass filter the negative voltage on the control grid 45 of the mixer 26 is rather high. However, when there is a severe disturbance anywhere in the entire frequency control system, the master captive oscillator breaks ont of lock, resulting in a loss of 50G-kc. input at the output of lter 25 (input to mixer 26), or in the decrease of this SOO-kc. signal to an unusable value, as explained hereinabove. Since frequency divider 29 is of the regenerative type, this loss of 50G-kc. input to such divider produces a marked change in the operation of such divider, as compared to the normal operation thereof. ln fact, the loss of 50G-kc. input to mixer 26 causes a marked drop in the negative bias voltage on control grid 45 ofthe mixer tube 26.

lt should be noted that in Fig. 3 a positive bias is applied to the cathode of tube 30, by means of a voltage divider including two fixed resistors 63 and 64 and an adjustable resistor 65, all connected in series between the positive terminal and ground, the said cathode being connected to the junction of resistors 64 and 65. The gain of the harmonic generator tube must be reduced, by means of this cathode bias, to prevent self-oscillation of the regenerative divider 29.

ln accordance with the first embodiment of this invention, the change 0f grid bias on `mixer stage 26, in response to the breaking-out of lock of master oscillator l (as the result of a transient disturbance, for example) is used to gate the output `of an auxiliary low frequency sweep oscillator 34 (for example, an audio oscillator operating at about 150 cycles) on or off, by means of bias connection 35.

The output of oscillator 34 is applied through a coupling capacitor 66 to the grid of a controllable oscillator gate 34', which is a sharp-cutoff vacuum triode. This triode is controlled from the grid voltage on control grid 45 of mixer 26 of the regenerative divider 29, by means of a connection extending from the lower end of leak resistor 67, through a resistor 68 to the bias connection which is coupled directly to grid 45 of the mixer tube 26. The output of gate 34 is applied to phase discriminator 28, by means of a connection from the anode of tube 34' through a capacitor 69 and a resistor 7G, to the phase inverter (phase discriminator) side of capacitor 42.

The high .negative bias on grid under normal frcquency control of oscillator 1, is applied to gate 34' and cuts off or closes this gate, the gate then functioning to cut off the low frequency sweeping voltage (output of oscillator 34) from the phase discriminator 28 when the 500 kc. input to divider 29 is stabilized by the lock-in of the master oscillator l. and the related locking-in of the regenerative frequency divider.

The lower negative bias on grid 45 in response to break-out of oscillator 1, is also applied to gate 34' and this opens this gate. the low frequency output of oscillator 34 then being applied through tube 34 to the phase discriminator 28.

Referring again to Fig. l, when gate 34' is thus opened the output of the low frequency sweep oscillator 34 is applied to the phase discriminator 28 by way of the connection including components 69 and 70, and this signal of about C. P. S. reappears at the output of the phase discriminator, since no reference, counteracting signal yof this frequency is applied to the phase discriminator. Thus, the output of `the auxiliary low frequency oscillator 34 reaches the reactance tube 2, causing the l50-cycle output of such oscillator (when the gate 34 is opened in the manner previously described) to sweep or modulate in frequency the master oscillator, either side of its nominal frequency.

The circuit of this first embodiment operates in the following manner: when the master. Oscillator 1 breaks out of lock for any reason (for example, as the result of a severe transient disturbance), the marked drop in bias voltage on grid t5 turns on gate tube 34', permitting the output of the auxiliary LF sweep oscillator 34 to sweep the master oscillator frequency either side of nominal. During this sweeping process the master oscillator frequency will go through such a frequency value that the said frequency, passing through the series of mixers and associated bandpass filters described, willie-establish or re-lock the frequency control and the master oscillator l will then be relocked in. The locking-in of the master oscillator results in a marl/ted increase of the negative voltage on grid 45, which biases off gate tube 34 and thus effectively stops the sweeping of the master oscillator. Conditions are then again normal.

The above-described sweep of the master oscillator frequency whenever such oscillator is not locked-in effectively enlarges many fold the lock-in range of the frequency control system. This is illustrated in Fig. 2, wherein the shaded rectangle B represents the extent of the lock-in range with sweep according to this invention, and this extent is many fold greater than the locl in range il, with no sweep. The dotted line A represents the hold-in range with sweep; it will be seen that this is of exactly the same extent as the hold-in range A, with no sweep. When a sweeping voltage is thus used according to this invention, even if there has been slow component `drift of oscillator 1 such that the reactance control volts has a value as represented by point C, and if a severe transient disturbance then occurs such as to cause oscillator l. to break out of lock, relocking will easily be effected since the point C is well within the enlarged lock-in range B resulting from the utilization of sweep according to this invention.

in another and preferred embodiment of the invention, now to be discussed in connection with Figs. 4 and 5, the auxiliary low frequency oscillator 34 and gate 34 are dispensed with and the regenerative frequency divider 29 itself is used as a source of auxiliary sweeping voltage. The frequency divider can be made to function in this manner by proper selection of LC values in the tuned circuits and of gain around the regenerative loop. In other words, in this embodiment the inherent self-oscillation of the regenerative frequency divider 29 (already available in the frequency control system disclosed) is utilized via the phase discriminator 28 to provide a low frequency sweeping signal for master oscillator 1, whenever frequency control has been lost, or when such oscillator is not locked in.

Now referring to Fig. 4, which is a par/tial (simplified) bloclc diagram of the preferred embodiment previously referred to, it may be seen that Fig. 4 is a simplified diagram of the 'l system, the auxiliary low frequency oscillator 34. the gate 34', and the bias control connection 35 being omitted from the Fig. 4 embodiment. Many of the circuits of Fig. l have been omitted from Fig. 4, except for the terminal connections, in order not to clutter the drawing unduly. The SOO-kc. signal from bandpass lter 2S is applied to the input of regenerative frequency divider 29 (which consists of units 26, 27, 3f) and 3i. arranged as in Fig. l), and the 50-kc. output of this divider is applied to phase discriminator 28, along with the SO-lcc. sawtooth reference frequency, which latter is applied to phase discriminator 28 by means of connection 33. The control voltage output of phase dscriminator 28 is applied to reactance tube 2, by means of a cathode follower stage as will be subsequently described, in order to control or vary (as Well as sweep) the frequency of the master (captive) oscillator 1.

Now referring to Fig. 5, which is a detailed schematic of the second (and preferred) embodiment of the invention, including regenerative frequency divider 29, phase discriminator 28, mixer 19 and filter 25, the output of bandpass filter 13, which is a signal within the 230-235 kc. range, is applied to the No. l grid of No. 4 mixer tube i? and the output of bandpass filter 24, which is a signal within the 265--270 kc. range, is applied to the suppressor grid 3:6 lof tube lil, which tube is a pentode vacuum tube. The two frequencies applied to tube 19 are mixed therein, and the beat (sum) frequency of 50() kc. appears at anode of this tube, as well as other beat frequencies.

The SDU-kc. frequency is selected from the output or anode circuit of mixer 19 by means of the bandpass filter Z5' tuned to pass 500 kc. and consisting of two resonant circuits tuned to 500 kc. and coupled together by means of a capacitor 38, one of these two resonant circuits (on one side of capacitor 33) being connected directly to anode 37 and the other of these two circuits (on the other side of capacitor S8) being connected directly to the suppressor' grid 39 of No. 6 mixer tube 26, which constitutes the first stage of regenerative frequency divider 29 and which is a pentode vacuum tube. Mixer tube 26 serves as a mixer and frequency multiplier. A resonant filter circuit 27, tuned to 50 kc. is connected directly to the anode 4l of tube 26. The 50G-itc. signal applied to grid 39 of tube 26 is in effect divided down to 50 kc. at the anode 4i of tube 26, and this 50-kc. signal (selectively passed by tuned circuit 27) is tai/.en off from anode "il by means of a coupling capacitor 42 and applied as one input to the phase discriminator 28. This 50-kc. signal passing through capacitor d?. constitutes the output of frequency divider 29.

The 5G-l csignal appearing at anode 41 is also applied through a coupling capacitor 4d to the control grid 43 of the frequency tripler tube 30, which also is a pentode vacuum tube. Tube 3d is biased so that it operates as a frequency tripler, producing a lSO-kc. signal from the SO-kc. signal input thereto, this 15G-lic. signal being selectively passed by means `of a resonant filter circuit Si connected directly to the anode of tube 3ft and tuned to ISO-kc. The ISO-kc. signal appearing at the anode of tube 3f) is applied through a coupling capacitor d4 to the control grid 45 of tube 26.

As previously stated, tube 26 functions in effect a frequency multiplier to multiply the 15G-kc. signal received from the anode of tripler Sti, by way of lter 3l, to a frequency `of 450 kc. which beats, in tube 26, with the SGO-kc. signal received from filter 2S, thereby producing a 50-kc. signal at anode 4l which is transmitted on to phase discriminator 23 for utilization therein.

In the regenerative frequency divider 29 as disclosed herein, the tripler 3@ drives the mixer 26 very hard5 gating the SOO-kc. input signal to tube 26 (from filter 25) at a lSO-kc. rate (the frequency ,of the signal at the anode of tube Sii, selected by filter 31), resulting in a strong 5ft-kc. component in the mixed anode current (of anode ill), and hence a Sli-kc. voltage across the 50-kc. tuned circuit 27. Thus, the regenerative frequency divider 2), during its normal operation, divides the 50G-lic. input signal applied to grid 39 down to a SO-kc. signal at the anode 4l (across tuned circuit 27), effecting a division by ten.

The Sil-kc. energy which-is selected by filter 27 is passed through coupling capacitor i2 to the grid 49 of a phase inverter triode 50.. The amplifier and phase inverter tube 56, to the grid of which the SO-kc. signal from mixer 26 and filter 27 is applied, amplities the Sli-kc. signal and provides SO-kc. balanced push-pull output which is utilized as signal input t-o the quadruple diode phase detector or phase discriminator 28, through coupling capacitors 51 and 52. The four diodes of the phase discriminator are denoted by numerals S3, 54, 55 and 56. One of the 50-kc. outputs of phase inverter 50 goes through capacitor 51 to the cathodes of 4diodes S3 and 55, While the other push-pull or antiphasal output goes through capacitor 52 to the anodes of diodes 54 and S6 and through a resistor 57 to the cathode of diode 53. For phase comparison with the approximately SO-kc. signal input from inverter 50, a standard or reference 50 kc. sawtooth-shaped input, derived from the frequency divider stage 8 of Figure 1, is applied by lead 33 to the cathode of diode 54 and to the anode or' diode in the phase detector or discriminator 28, the D. C. control output results from the phase comparison of the Sil-kc. sawtooth reference wave and the SO-kc. signal from inverter 50.

The D. C. control output of phase discriminator 23 is taken from the joined-together cathode of diode 56 and the anode of diode 55. Filtered by the capacitor S, the phase discriminator output is direct coupled to the grid 59 of a triode 60 connected as a cathode follower amplier stage. A resistor 62 is connected from the cathode 61 of tube 60 to ground.

From the cathode 61 of the cathode follower', the D. C. control output of the phase discriminator 28 is applied to the grid of the reactance tube 2 to control the frequency of the master oscillator 1. In this way, during normal operation of the transmitter-receiver of this invention, the phase discriminator 28 very accurately controls the frequency of master oscillator l, a correcting voltage out of the phase discriminator appearing whenever oscillator 1 tends to drift in frequency; the frequency of this oscillator is then maintained very accurately at its proper value bythe phase comparison with a highly stable crystal controlled frequency. The phase discriminator just described has the hold-in range A and the lock-in range B already described in connection with Figure 2.

The characteristics of the regenerative frequency divider 29, comprising No. 6 mixer 26 and tripler stage 30, are such that under normal frequency control of oscillator 1 (when conditions are normal, that is, when oscillator 1 is being held in control so that there is a 50G-kc. signal of the proper magnitude at the output of bandpass lter 25) the voltage on the control grid 43 of the tripler 30 is relatively high, a negative voltage of approximately twenty volts then being developed here. However, when there is a severe disturbance anywhere in the entire frequency control system, the master captive oscillator breaks out of lock, resulting in a loss of SOO-kc. input at the output of filter 25 (input to mixer 26), or in the decrease of this SOO-kc. signal to an unusuable value, as

explained hereinabove. Since frequency divider 29 is of the regenerative type, this loss of SOO-kc. input to such divider produces a marked change in the operation of such divider, as compared to the normal operation thereof. In fact, the loss of SOO-kc. input to mixer 26 causes a marked drop in the bias voltage on control grid 43 of the tripler tube 30, to approximately four volts negative.

The components in the LCtuned circuit 27 have such values that this circuit has a Q on the order of 35, while the components in the 15G-kc. tuned circuit or tank 31 have such values that this circuit has a Q on the order of 50. It may be stated that the tripler or harmonic generator tube 30 has the output voltage developed across its tank circuit 31 coupled by the mixer 26 and the relatively broad SO-kc. tank 27 back to the grid 43 of the former, completing the feedback loop. This feedback loop provides sutlicient gain to start and sustain oscillation with a frequency approximating 15G-kc. with a screen grid voltage on tube 30 as low as three or four volts. As the screen grid voltage on tube 30 (and the tube transconductance) is increased, the oscillation described takes on the nature of a self-quenched super-regenerative oscillation, in which the RC time constant of the grid circuit 46, determines the relaxation or quenching frequency.

As the C of capacitance 40 is raised, the relaxation quenching frequency is lowered. The resulting oscillatory energy thus developed in divider 29 is spread over a wide frequency spectrum. Components of this oscillatory energy which shock-excite the SO-kc. tank circuit 27 are passed on to the phase discriminator 2; as one of the inputs thereto.

In phase discriminator 28, the unstable fluctuating output frequency out of divider 29 (under these conditions) is compared with the stable SO-kc. reference frequency from divider S, resulting in a fluctuating voltage output (actually, a random beat frequency, since the` relaxation oscillation consists of various random frequencies) which is supplied to the reactance tube 2. This fluctuating voltage output out of discriminator 28 has a predominant low frequency component which, applied to the reactance tube 2, produces a fluctuation of the master oscillator 1 around its normal frequency. Thus, in this embodiment the regenerative divider 29 itself is used as a source of the auxiliary voltage which sweeps or tluctuates the frequency of the master oscillator. As a result of this fluctuation or sweep of the master oscillator frequency, the beat frequency out of mixer 19 is fluctuatcd or swept, and at some time during this fluctuation the frequency of the master oscillator is re-locked so that frequency control is re-established. The locking-in of the master oscillator, or the re-establishing of the frequency control, results in the reappearance of the 50G-kc. signal at the input of divider 29, driving this divider, producing a high negative voltage on grid 43 and cutting off the relaxation oscillation. The sweeping or fluctuation of the master oscillator frequency then stops and the tubes 26 and 30 function normally, as a frequency divider.

To briey summarize the foregoing operation, when the master oscillator 1 is locked in the frequency divider 29 functions as such, developing a high negative voltage on grid 43 which prevents any relaxation oscillation. When the oscillator 1 breaks out of lock, the SOO-kc. input signal to the divider fails and the negative voltage on grid 43 drops, causing a relaxation oscillation to start in regenerative divider 29, which in turn produces a fluctuation of the master oscillator frequency during which the oscillator again locks in. When it does so, the relaxation oscillation is cut olf and the regenerative frequency divider again operates normally.

The sweep or iluctuation of the master oscillator frequency whenever the oscillator is not locked in (using the frequency divider itself as the source of auxiliary sweeping voltage, in the system of Figs. 4-5) effectively enlarges many fold the lock-in range of the frequency control system, just as previously explained in connection with Figs. 1 and 2. In Fig. 2, the rectangle B represents the extent of the lock-in range with sweep according to Fig. 4, and it will be seen that this is much greater in extent than the lock-in range B with no sweep.

In another embodiment of the invention the regenerative frequency divider 29"used as a source of auxiliary sweeping voltage, is of the more conventional type, similar to Fig. 3. In this arrangement the frequency multiplier 30 multiplies by nine times the 50 kc. input thereto, instead of the tripling action described in connection with Figs. 4-5. The output of the frequency multiplier 30 is fed to a filter 31 tuned to the ninth harmonic or 450 kc. (as in Fig. 3) and the mixer 26 is not required to perform any frequency multiplication. Again, in this arrangement (as in Figs. 4 5) the choice of circuit constants and gain around the feedback loop permits the development of a relaxation oscillation when the master oscillator is out of lock The resulting uctuating voltage output of phase discriminator 28 sweeps the master oscillator to reestablish lock-in.

The following are representativey values for a circuit built according to Fig. 3 and successfully tested.

The following are representative Values for a circuit built according to Fig. 5 and successfully tested.

Tube 26 5636.

Tube 30 5840.

Resistor 46 lmegohm.

Capacitor 38 l0 mmfd.

Capacitor 40 22 mmfd.

Capacitor 42 22 mmfd.

Capacitor 44 22 mmfd.

lnductance of ckt. 27 9.5-10 mh. (adjustable). l'nduotance ofckt. 31 9.5-10 mh. (adjustable).

What is claimed is:

l. In combination, an oscillator the frequency of which is to be controlled, a regenerative frequency divider coupled to the output of said oscillator, means coupled to the output of said divider for locking the frequency of said oscillator to a predetermined frequency established by a stable frequency source, said means having a normal lock-in range of limited extent over which the oscillator frequency is initially locked in by said means, said divider being constructed and arranged to develop a voltage change therein in response to the breakingout of lock of said oscillator frequency, and means responsive to said voltage change for sweeping said oscillator frequency through a range greater than said normal lock-n range to re-establish frequency control of said oscillator.

2. In combination, an oscillator the frequency of which is to be controlled, a regenerative frequency divider coupled to the output of said oscillator,` and means coupled to the output of said divider for locking the frequency of said oscillator to a predetermined frequency r established by a stable frequency source, said means having a normal lock-in range of limitedextent over which the oscillator frequency is initially locked in by said means, said divider being constructed and arranged to develop therein a wave of random frequency in response to the breaking-out of lock of said oscillator frequency, the application of the random frequency wave output of said divider to said locking means causing corresponding frequency modulation of said oscillator.

3. ln combination, an oscillator the frequency of which is to be controlled, voltage-responsive frequency controlling means coupled to said oscillator, a regenerative frequency divider coupled to the output of said oscillator, a phase discriminator having two inputs, means applying the output of said divider as one input to said discriminator, means for applying a Wave of stable frequency as the other input to said discriminator, means for applying the voltage output of said discriminator to said frequency controlling means, the combination of said phase discriminator and said frequency controlling means providing a normal lock-in range of limited extent over which the oscillator frequency is initially locked in, said divider being constructed and arranged to develop a voltage change therein in response to the breaking-out of lock of said oscillator frequency, and means responsive to said voltage change for sweeping said oscillator frequency through a range greater than said normal lock-in range.

4. In combination, an oscillator the frequency of which is to be controlled, voltage-responsive frequency controlling means coupled to said oscillator, a regenerative frequency divider coupled to the output of said oscillator, a phase discriminator having two inputs, means applying the output of said divider as one input to said discriminator, means for applying a wave of stable frequency as the other input to said discriminator, and means for applying the voltage output of said discriminator to said frequency controlling means, the combination of said phase discriminator and said frequency controlling means providing a normal lock-in range of limited extent over which the oscillator frequency is initially locked in, said divider being constructed and arranged to develop therein a wave of random frequency in response to the breaking-out of lock of said oscillator frequency, the application of the random frequency Wave output of said divider to said phase discriminator causing corresponding frequency modulation of said oscillator,

5. In combination, an oscillator the frequency of which is to be controlled, a regenerative frequency divider coupled to the output of said oscillator, means coupled to the output of said divider for locking the frequency of said oscillator to a predetermined frequency established by a stable frequency source, said means having a normal lock-in range of limited extent over which the oscillator frequency is initially locked in by said means, said divider being constructed and arranged to develop a voltage change therein in response to the breaking-out of lock of said oscillator frequency, an auxiliary oscillator operatively arranged to sweep the frequency of said controlled-frequency oscillator through a range greater than said nonnallock-in range, and means coupling said auxiliary oscillator to said divider to render operative said oscillator in response to said Voltage change. l 6. In combination, an oscillator the frequency of which is to be controlled, voltage-responsive frequency controlling means coupled to said oscillator, a regenerative frequency divider coupled to the output of said oscillator, a phase discriminator having two inputs, means applying the output of said divider as one input to said discriminator, means for applying a wave of stable frequency as the other input to said discriminator, means for applying the voltage output of said discriminator to said frequency controlling means, the combination of said phase discriminator and said frequency controlling means providing a normal lock-in range of limited extent over which the oscillator frequency is initially locked in, said divider being constructed and arranged to develop a voltage change therein in response to the breaking-out of lock of said oscillator frequency, an auxiliary oscillator operatively arranged to sweep the frequency of said controlled-frequency oscillator through a range greater than said normal lock-in range, and means coupling said auxiliary oscillator to said divider to render operative such oscillator in response to said voltage change.

7. in combination, an oscillator the frequency of which is to be controlled, at least one mixer excited by Waves from said oscillator and by waves of stable frequency to produce beat frequency resultant waves, a regenerative frequency divider coupled to the output of said mixer, means coupled to the output of said divider, and responsive to variations in the frequency of said resultant waves from a predetermined value, for controlling the frequency of said oscillator, and means responsive to a voltage change occurring in said divider in response to the decrease below a usable amplitude level of the resultant waves applied to said divider, for sweeping the frequency of said oscillator through a certain range.

8. In combination, an oscillator the frequency of which is to be controlled, at least one mixer excited by waves from said oscillator and by Waves of stable frequency to produce beat frequency resultant waves, a 'regenerative frequency divider coupled to the output of said mixer, means coupled to the output of said divider and responsive to variations in the frequency of said resultant waves from a predetermined value, for controlling the frequency of said oscillator, an auxiliary oscillator operatively arranged tosweep the frequency of said controlled-frequency oscillator through a certain range, and means coupling said auxiliary oscillator to said divider to render operative such oscillator in response to a voltage change occurring in said divider in response to the decrease below a usable amplitude level of the resultant waves applied to said divider.

9. ln combination, an oscillator the frequency of which is to be controlled, at least one mixer excited by waves from said oscillator and by waves of stable frequency to produce beat frequency resultant waves, means responsive to variations in the frequency of said resultant Waves from a predetermined value for controlling the frequency of said oscillator, a regenerative frequency divider coupled to said mixer, said divider operating to develop therein a wave a random frequency in response to the decrease below a usable amplitude level of the resultant waves applied to said divider, and means coupling said divider to said controlling means to frequency modulate said oscillator by output produced by said random frequency Wave.

l0. In combination, an oscillator the frequency of which is to be controlled, voltage-responsive frequency controlling means coupled to said oscillator, at least one mixer excited by waves from said oscillator and by waves of stable frequency to produce beat frequency resultant waves, a phase discriminator having two inputs, means including a regenerative frequency divider coupled between said mixer and said discriminator for applying waves representative of said beat frequency resultant waves as one input to said disciiminator, means for applying waves of stable frequency as the other input to said discriminator, means for applying the voltage output of said discriminator to said frequency controlling means, and means responsive to a voltage change occurring in said divider in response to the decrease below a usable amplitude level of the resultant waves applied to said divider, for sweeping the frequency of said oscillator through a certain range.

ll. In combination, an oscillator the frequency of which is to be controlled, voltage-responsive frequency controlling means coupled to said oscillator, at least one mixer excited by waves from said oscillator and by waves of stable frequency to produce beat frequency resultant waves, a phase discriminator having two inputs, means including a regenerative frequency divider coupled between said mixer `and said discriminator for applying Waves representative of said beat frequency resultant waves as one input to said discriminator, means for applying waves of stable frequency as the other input to said discriminator, means for applying the voltage output of saiddiscriminator to said frequency controlling means, an auxiliary oscillator operatively arranged to sweep the frequency of said controlled-frequency oscillator through a certain range, and means coupling said auxiliary oscillator to said divider to render operative such oscillator in response to a voltage change occurring in said divider in response to the decrease below a usable amplitude level of the resultant waves applied to said divider.

12. In combination, an oscillator the frequency of which is to be controlled, voltage-responsive frequency controlling means coupled to said oscillator, at least one mixer excited by waves from said oscillator and by waves of stable frequency to produce beat frequency resultant waves, a phase discriminator having two inputs, means for applying waves representative of said beat frequency resultant waves as'one input to said discriminator, means for applying waves of stable frequency as the other input to Said discriminator, means for applying the voltage output of said discriminator to said frequency controlling means, a regenerative frequency divider coupled to said mixer, said divider operating to develop therein a wave of random frequency in response to the decrease below a usuable amplitude level of the resultant Waves applied to said divider, and means coupling said divider to said phase discriminator to frequency modulate said oscillator by output resulting from said random frequency Wave.

References Cited in the le of this patent UNITED STATES PATENTS 2,287,925 White June 3U, 1942 2,434,293 Stearns Jan. 13, 1948 2,434,294 Ginzton Ian. 13, 1948 2,452,575 Kenny Nov. 2, 1948 2,541,454 White et al. Feb. 13, 1951 2,595,608 Robinson et al. May 6, 1952 2,698,904 Hugenholtz Jan. 4, 1955 

